A quadrature modulation system, which produces a transmit signal by modulating two quadrature carriers in accordance with an input signal comprising an in-phase component signal and a quadrature component signal, can flexibly achieve a variety of modulation schemes and signal constellations, and is therefore employed in many communication apparatuses and electronic appliances.
FIG. 33 is a diagram schematically showing the configuration of such a quadrature modulation system. The input signal to the quadrature modulation system 1, which is transmit data such as a complex baseband signal, is made up of an in-phase component signal and a quadrature component signal, and they are input to the quadrature modulation system 1 via an I channel and a Q channel respectively corresponding to the two carriers.
These input signal components are converted by D/A converters 13I and 13Q, provided in the I and Q channels respectively, into analog signals for the respective channels. Then, the transmit signal is formed by modulating the two carriers with the respective analog signals in a quadrature modulator 14, and the thus formed transmit signal is fed via a power amplifier 15 to an antenna (not shown) for transmission.
In such a quadrature modulation system, when frequency-converting the transmit signal, i.e., the complex baseband signal, by the analog quadrature modulator (QMOD), a DC offset may be added to the transmit signal in analog device circuits in the quadrature modulation system as a whole, for example, in the analog device circuits between the quadrature modulator 14 and the digital/analog converters 13I and 13Q (hereinafter called the “D/A converters”), due to differences or variations in the characteristics of multiplier circuits in the analog domain.
This DC offset manifests itself as a carrier leakage (unwanted carrier) in the frequency-converted analog transmit signal, causing an adjacent channel leakage and thus leading to a degradation of the transmit signal quality.
In one method practiced in the prior art to compensate for the DC offset, a component inverse in polarity to the DC offset expected to be added during the process between the D/A converters 13I and 13Q and the quadrature modulator 14 is added in advance to the transmit signal before input to the D/A converters 13I and 13Q, thereby canceling out the DC offset.
To generate such a DC offset canceling signal, there is proposed a method in which a portion of the quadrature-modulated transmit signal is fed back and the feedback signal is analyzed to measure and correct the DC offset (refer, for example, to patent document 1 listed below), and also a method in which the transmit signal is subtracted from the feedback signal to extract an error component and then the DC offset is measured and corrected.
FIG. 34 is a diagram showing a configuration example of a quadrature modulation system in which the offset compensation is performed by generating a DC offset canceling signal. In this configuration example, the transmit signal is subtracted from the feedback signal to extract an error component, and then the DC offset is measured and corrected.
For this purpose, a directional coupler 16 is inserted between the power amplifier 15 and the antenna (not shown), and a portion of the transmit signal is fed back through a monitor terminal of the directional coupler 16. The transmit signal thus fed back is passed through a mixer 82, an analog/digital converter (hereinafter called the “A/D converter”) 83, and a quadrature demodulator 84 to generate quadrature monitored signals i and q, which are supplied to a DC offset correction value estimating unit 20.
Then, based on these quadrature monitored signals i and q and the earlier described input signal components, the DC offset correction value estimating unit 20 estimates DC offset correction values for compensating for the DC offset for the in-phase and quadrature components, respectively.
The DC offset correction values thus estimated are added by adders 12I and 12Q respectively to the in-phase and quadrature input signal components before input to the respective D/A converters.
An output signal of an oscillator 81 is input to the mixer 82 through its local oscillator input terminal, and the transmit signal separated by the directional coupler 16 is mixed with the local oscillator signal for frequency conversion to produce an intermediate frequency signal.
The A/D converter 83 converts the intermediate frequency signal into a digital signal that is synchronized to a clock signal of a given frequency.
The quadrature demodulator 84 quadrature-demodulates the digital signal to produce the quadrature monitored signals i and q corresponding to the I and Q channels, respectively, that are in phase quadrature to each other.
The DC offset correction value estimating unit 20 obtains offset components contained in the quadrature monitored signals i and q, for example, by smoothing these signals in the complex plane. Likewise, the DC offset correction value estimating unit 20 obtains offset components contained in the in-phase and quadrature input signal components by smoothing them in the complex plane.
Then, the DC offset correction value estimating unit 20 extracts only the offset component added in the quadrature modulation system 1 by subtracting the in-phase signal component in the quadrature monitored signal i and the quadrature signal component in the quadranture monitored signal q from the respective input components I and Q, and estimate the inverse offset obtained by inverting the sign of the offset component as the DC offset correction value.
Patent document 1: Japanese Unexamined Patent Publication No. H10-79692
Patent document 2: Japanese Unexamined Patent Publication No. 2001-237723